Minun on löydettävä joku toinen.

English Translation

I have to find another one.

Is this relationship-related? I have to find another girlfriend/boyfriend?

I don’t think it’s exclusively for that purpose. I can imagine someone saying “Minun on löydettävä joku toinen opettaja.” Maybe it could even refer to an object.

I think that is right. I tend to think of ‘joku’ as referring to people but it doesn’t have to.